DE19958211A1 - Diffraction grating with enhanced "blaze" power at two wavelengths - Google Patents

Abstract

An Echelle grating has a groove facet angle and groove periodicity, so that for laser-generated light of two different wavelengths the highest propagating diffraction order is "blazed" and the next higher order is not propagating, so that the intensity of each blowing order is increased. In one example, the 84th diffraction order from a KrF excimer laser emitting at 248.4 nm and the 108th diffraction order from an ArF excimer laser emitting at 193.3 nm are both "blazed" when the groove spacing d is 010623 millimeters, which corresponds to a frequency f of 94.13 grooves per millimeter, and wherein the groove facet angle THETA¶g¶ is approximately 80 °, the next higher order for each laser, i.e. H. the 85th and 109th orders are not spreading and thus both "Blaze" orders are reinforced.

Description

The present invention relates to optical gratings for diffraction of Rays of light, especially on Echelle grids, and especially on Littrow oriented Echelle grating with "Blaze" performance on two different worlds length increased by "passing-off" or suppression the next higher diffraction orders at each wavelength.

A diffraction grating is an arrangement of reflective or transmissive elements that are at a distance from the examined light world len lengths are comparable spaced, such as a regular Patterns of equally spaced, transparent slits or openings in an opaque screen or collection of reflective grooves or grooves on a substrate. An electromagnetic wave that strikes the If the grating hits, its electric field strength or phase becomes after the diffraction or modified both in a predictable manner.

The present invention relates to electromagnetic radiation, in in this context referred to as "light", which is almost completely mono is chromatic, such as that emitted by an excimer gas laser and further relates to the diffraction of radiation by linear gratings with triangular grooves, each groove having first and second facets that themselves at a given angle to each other and to the longitudinal axis or Level of the grid. Such a triangular lattice gives a step-like appearance in cross-section.

When monchromatic light strikes a grating surface, the light is diffracted in discrete angular directions. Each grid groove can be interpreted as a very small, slit-shaped source of diffracted light. The light elementary wave diffracted by each groove interferes with the light from other grooves to form a diffracted wavefront. The utility of gratings is related to the fact that a certain set of discrete angles exist, known as "orders", in which, for a given distance d between the grooves, the diffracted light elementary wave from each groove facet is in phase with the Elementary waves that are diffracted at every other groove facet, so that the elementary waves interfere constructively. Only the light that falls on the grating with an angle of incidence α and is diffracted along a diffraction angle β for which the groove spacing d multiplied by the sum of the sine of α and β is equal to an integer multiple m of the wavelength λ is structurally diffracted, whereby m is therefore the constructive diffraction order number. These parameters are related to each other in the grid equation:

mλ = d (sin α + sin β) (equation 1)

The present invention relates to a class of linear diffraction gratings, which are known as Echelle grids or Echelles. Such grids have one first and second facets, which have a different length, the Ratio of lengths is typically between approximately 2: 1 and 6: 1, about 4: 1 is common in the art. This ratio is as the r number of a grid known; thus, for example, an r-4 grid has egg NEN, or short facet angle of 75 ° 58 'with respect to the longitudinal axis of the grid. Furthermore, an echelle generally indicates that the beam and the "blaze" diffracted beam on the same side of the plane perpendicular to the longitudinal axis of the grid. Echelles are relatively crude, with Precision-made gratings, generally at high diffraction angles and can be used for spectral orders with a high order number. A typical groove spacing is between about 20 and about 300 grooves per millimeter. The angles of incidence are typically between approximately 63 ° (r-2) and approximately 80 ° (r-6) with respect to the grid normal and  perpendicular and furthermore the spectral orders used lie between about 10 and 500, typically about 100. Echelles are ins especially in applications that require a high degree of optical dispersion and require resolution, beneficial.

The intensity of the light, or the amount of energy radiated in each diffraction order is included and in relation to the intensity of the one falling beam is scaled as the yield or efficiency the diffraction of this order. The diffraction orders of a grating contain unequal amounts of energy and for all given light wavelengths most orders contain very low energies. For each grid yes, and for any wavelength that has a given angle of incidence occurs, there is an order that contains the highest amount of diffraction energy, this in the art as the "Blaze" order or preferred reflections on is known. Orders that are angularly on both sides of the "blaze" - Order (m + 1 and m - 1) are spaced, may or may not contain a significant amount of energy.

Orders that are able to emit diffracted light are those where the diffracted beam can emerge from the grating. Orders that are angular in directions extending into the grating are referred to in the art as "below the horizon" and cannot propagate diffracted light. An Echelle grating, which is specifically oriented so that the beam diffracted in the "Blaze" order and the incident beam have the same path in space (angle of incidence θ _i is equal to the diffraction angle θ _d ) is located in the so-called " Littrow "configuration. For a Littrow-oriented grating, the angle of the "blaze" order θ _{B is} approximately equal to the facet angle θ _G , which is equal to the arctangent of the grating number r. In this configuration, the lattice equation (Equation 1) can be reformulated to:

mλ = 2d sin θ _B (Littrow) (Equation 2)

It is an interesting property of the "Littrow" -configured Echelle- Grid that if the angle of incidence is set so that the "blaze" angle is the highest numbered, radiating or spreading order, the Ord a little more energy than in another angular position and more than would have been predicted by the scale theory, contains what is in the Fachge is referred to as the "anomaly condition". Radiation from this "blaze" - Angle is called "reinforced". Since the energy conservation law applies, the energy that is diffracted into non-propagating orders would turn around distributed to the spreading orders, of which the neighboring "Blaze" - Order gets the most energy allocated.

For example, an Echelle grating can be used as an optical element be useful in a gas laser cavity or gas laser resonance room, the echelle forming one end of the cavity and passing around an axis the Echelle and perpendicular to the laser beam can be rotated around the Laser to tune to the wavelength with the maximum resonance. In particular their gas lasers are not completely monochromatic due to, for example of Doppler effects from the speed distribution of the accumulation of gas molecules during photon emission results. In some La Water applications, such as in the manufacture of computer switches orbiting, it is very desirable to include the spectral output of the laser tight. This can easily be achieved by using a hochzer scattering Echelle grating, used in the Littrow configuration that the can suppress external wavelengths of the laser distribution and in the Cavity bends for reinforcement of a substantially constricted or narrow wavelength band. This is known in the art as "spectral line narrowing ". A suitable grid for this application is an r-5 grid, such as the 35-13 - * - 406 grid, which has 85.84 grooves per millimeter and can be purchased from Richardson Grating Laboratory, an Ab division of Spectronic Instruments, Inc., Rochester, New York, USA  

It is also very desirable that the "Blaze" order, which applies to La tuning or spectral line narrowing is available, the highest numbered, spreading order and thus reinforced what is as "supra" is described. As is known in the art, a grid, like the Richardson grating described above with a given spacing d, be oriented so that the reinforcing "Blaze" order is only for one sin Gular wavelength is available. For example, this can be 85.84 creasing Littrow-mounted per millimeter grid, so that the "Blaze" order (92) of light with a wavelength of 248.4 nm as it is from a krypton Fluorine (KrF) excimer laser is produced that is spreading and the next higher order (93) is not spreading. This grid is also for the Spectral line narrowing of an argon flourine (ArF) excimer laser with 193.3 nm useful, but the "Blaze" order (in the Littrow configuration) won't reinforced. In the art, a different Littrow-mounted would Echelle grating is designed to exhibit "blazing" at 193.3 nm, but not do at 248.4 nm.

Diffraction gratings are very difficult, expensive, and time-consuming to manufacture and there are only a few "ruling" machines in the world directions for making diffraction gratings that are capable of high to produce qualitative grids. So it is very desirable to have an Echelle Grid that is capable of reinforcing "blaze" behavior to show two different light wavelengths.

It is a first object of the invention to provide an improved Echelle diffraction grating to provide which increased "blaze" diffraction of laser light with two ver different wavelengths in two different diffraction orders shine.

It is another object of the invention to provide an improved echelle Diffraction grating, which is capable of enhanced "blaze" diffraction of light  of both a KrF excimer laser and an ArF excimer laser demonstrate.

It is still another object of the invention to provide an improved method of loading adjustment of the optimal groove spacing and the groove facet angle for have an Echelle diffraction grating for enhanced "blaze" diffraction of laser light at two different wavelengths in two different To allow diffraction orders.

Briefly described, an Echelle grating according to the present invention has a specific groove spacing d and groove angle θ _g which the equation 2 within an acceptable small range for two different laser light wavelengths λ ₁ and λ ₂ and two different corresponding diffraction orders m ₁ and m ₂ fulfilled, where both orders are "Blaze" orders and the next higher order is not spreading or is not propagating. A known KrF laser essentially emits at 248.4 nm and a known ArF laser essentially emits at 193.3 nm. Through a new process, designated "infra", we have recognized that the 84th order of the KrF The laser and the 10th order of the ArF laser both "blaze" or preferably reflect when d is 0.10623 mm, which corresponds to a frequency f of 94.13 grooves per millimeter, and where θ _{g is} approximately 80 ° (r According to equation 2, m _ArF λ _ArF must be equal to m _KrF λ _KrF By multiplying with the appropriate values it is recognized that m _ArF λ _ArF = 20 866 nm and m _KrF λ _KrF = 20 876 nm. The agreement of 100 millionths is sufficient to enable both orders to "blaze". In addition, the method shows that the next higher orders for each laser, for example the 85th or 109th order, are not spreading and thus both "blaze" - Orders are strengthened.

The foregoing and other objects, features and advantages of the invention as well as currently preferred embodiments thereof are described by the  Read the following description in connection with the attached drawing The drawings show:

Fig. 1 is a cross section through a linear triangular diffraction grating;

Fig. 2 is a cross section of an Echelle diffraction grating;

Fig. 3 is a graph of the calculated diffraction efficiency is plotted against the Rillenfacettwinkel for the "Blaze" order 122 of ArF excimer laser, which emits light at 193.3 nm when it by a process known from the prior art echelle grating is flexed at 83.14 grooves per millimeter;

Figure 4 is a graph of the calculated diffraction yield versus groove facet angle for the "blaze" order 95 of a KrF excimer laser which emits 248.4 nm light when emitted by an Echelle known in the art. Grid is broken with 83.14 grooves per millimeter;

Fig. 5 is a graph as shown in Fig. 3 for an Echelle grating in accordance with the invention having an enhanced "blaze" order 108 when flexed by a grating at 94.13 grooves per millimeter;

Fig. 6 is a graph as shown in Fig. 4 for an Echelle grating in accordance with the invention having an enhanced "blaze" order 84 when flexed by a grating at 94.13 grooves per millimeter;

Fig. 7 is a graph of the calculated order number m ₁ plotted against the relative "blaze" angle for the condition that θ _ArF <θ _KrF ; and

Fig. 8 is a graph of the calculated order number m ₁ plotted against the relative "blaze" angle for the condition that θ _ArF <θ _KrF .

Referring to Fig. 1 is a schematic linear Reflektionsbeu supply grid is shown in cross section, of a reflective material, for example aluminum or glass with vapor-deposited aluminum, as is known in the art, formed 10 with triangular grooves. The grid 10 has a plate-shaped body 12 with a planar rear surface 14 which defines a grid reference surface. A front surface, opposite the rear surface 14 , has features with a plurality of highly parallel triangular grooves 16 formed by the edge portions of first and second facets 18 and 20 , respectively. The facets preferably meet at their intersection points 19 with an opening angle 17 comprising almost 90 °. The facets 20 are inclined with respect to the reference surface 14 at an angle θ _g and the grooves have a period d. A light beam 22 is incident on all second facets 20 at an angle α, measured from the grating normal plane 24 , which is perpendicular to the reference surface 14 , and is diffracted at many angles of order, the only one shown being the angle β, which is also from the grid normal 24 is measured starting, which is also the "blaze" direction in the example shown. The facet normal 26 divides the included angle between the incident and the diffracted rays. The "Blaze" order spread is in the direction indicated by an arrow 28 .

Referring to Figure 2, an Echelle grating 30 is shown in the Littrow configuration, where α = β and both the incident beam and the diffracted elementary wave coincide with the facet normal.

When grating 30 is provided with ad of, 012028 (83.14 grooves per centimeter), θ _g of approximately 79 °, and a groove opening angle of approximately 85 °, "Blazed" is the 95th order of a KrF laser with a yield of about 75% as shown in FIG. 4. The 96th order is not spreading and the "blaze" of the 95th order is thereby increased. With the same grating, "Blazed" the 122nd order ArF laser with a yield of approximately 72% as shown in Figure 3; however, the 123rd order is spread as well and thus the "blazing" of the 122nd order is not amplified.

As described with "supra", if the grille 30 according to the invention is provided with a d of, 010623 mm (94.13 grooves per millimeter), θ _g of approximately 80 ° and a groove opening angle of 85 °, "Blazed" 84th order of a KrF laser with a yield of approximately 73% as shown in Fig. 6 and the 85th order is canceled; and the 108th order of an ArF laser "Blazed" with a yield of about 80% as shown in Fig. 5, and the 109th order is canceled. Thus, a simple grating according to the invention is capable of providing an enhanced "blaze" at a variety of incident wavelengths.

Our process for determining the design parameters for one with two Wavelength, reinforced "blazing" grating is as follows.

First, a Littrow geometry is chosen for mounting the grating, the "blaze" angle θ _{B being} equal to the angle of incidence α and the angle of diffraction β. A starting value for the groove spacing d is then selected so that the "blaze" angle θ _B is substantially equal to the rilfen angle θ _G (the "blaze" beam follows the facet normal).

Second, the two wavelengths of interest, λ ₁ and λ _{2, are} determined and λ ₁ is assigned to the shorter wavelength.

Third, a range of order numbers m _{1 to be used} for the shorter wavelength is selected, which is around 100, for example 80 <m <140. These imposed values of m ₁ are all integers.

Fourth, it is assumed that θ _{B will be} similar but not identical for both wavelengths, and values for m ₁ and m ₂ in the range for θ _{B λ 1} / θ _{B λ 2} closest to 1.0 is determined as follows. According to equations 1 and 2, sinθ _{g is} proportional to mλ, and thus where θ _B1 <or <θ _B2 , m ₁ λ ₁ <or <m ₂ λ ₂ . m ₂ λ ₂ / m ₁ λ ₁ can be plotted as a function of the possible values of m ₁ . For values of θ _B1 <θ _B2 , m _{2 is} searched as the integer value below m ₁ λ ₁ / λ2, and for values for θ _B1 <θ _B2 , m ₂ is searched as the integer value below (m ₁ λ ₁ / λ ₂ ) + 1 wanted. These values for m ₁ for which m ₂ λ ₂ / m ₁ λ _{1 is} closest to 1.0 are then noted for further use.

Fifth, the groove frequency f (= 1 / d) is calculated for every order m ₁ (from step 4), where the next higher orders do not spread:

f = 1 / {(m ₁ +1) λ ₁ - m ₁ λ _1/2} (Equation 3)

Note: It is desirable to have a small constant value for the kür add several wavelengths to each of these groove frequencies to be sure deliver that the next higher order lies behind the horizon (<90 °) and therefore cannot spread. If the "passing-off" order for the short re wavelength is ensured, the "passing-off" order for the longer Wavelength also ensured.

Sixth, the difference of the diffraction angles θ _{B λ 1} -θ _{B λ 2} for each groove frequency is calculated from step 5 and the groove frequency is selected from which is associated with the smallest positive difference of the diffraction angles. The geometric parameters were thus determined for an Echelle grating which will show increased "blazing" at both λ ₁ and λ ₂ .

example

The groove frequency for an r-5 (θ _g = 80 °) 85 ° aperture angle Echelle grating, which increases "blazing" for both, an ArF excimer laser (λ ₁ = 193.3 nm) and a KrF excimer Laser (λ ₂ = 248.4 nm), was determined.

Possible values for m ₁ between 94 and 124 were examined in accordance with step 4 above and shown as recorded in FIGS. 7 and 8. A reasonable choice for the operational order numbers at 193.3 nm were the m ₁ order numbers that were closest to 1.0 with the values 32 of m ₂ λ ₂ / m ₁ λ ₁ : 95, 100, 103, 104, 108 , 109, 112, 113, 117, 118, 121 and 122.

The calculated results after following the process steps of "su pra" are shown in the following table:

It can be seen that the optimal grating frequency f 0.09413 grooves per micro meter (94.13 grooves per millimeter), with a "blaze" - Angle difference of only + 0.155 ° between the optimal angles for both Laser and sees an enhanced "blaze" angle in the 108th order for the ArF laser and in the 84th order for the KrF laser. However, it is understand that this is not a unique solution, but the optimal one for an installation that has a fixed angle of incidence α. For example, is to see that the grid, with f = 0.08702 and a "blaze" -  Angular difference of +0.162, the blaze in orders 117 and 91 reinforced.

A triangular lattice according to the invention can be made by known methods ren and devices are formed. A main grid or master grid can by surface pressing or embossing in an aluminum substrate by egg ne "ruling" machine can be generated. Such a grid is usually not Can be used directly for diffraction, since material that is formed during the formation of the Grooves shifted was accumulated on the outer groove vertices is, the inner groove vertices by the "Ruling" tool ex be formed. Typically, a first metalized (first genes ration) cast with an inverted topography that is fully functional is competent. Another metalized cast (second generation) is thus a duplicated master for casting copies of the third generation, the are also fully functional and with commercial quantities can be manufactured and sold. Thus all copies point to an unused one generation invert an inverted topography.

An advantage of the Echelle grating according to the invention is that it is easy on their flatness can be tested by known techniques ken use a known helium neon (HeNe) laser, the light with 632.8 nm, which is on the grating within one degree of that intended use angle of incidence, preferably at approximately α = 79.36 °, comes up with.

From the previous description, it will be obvious that a improved Echelle diffraction grating was shown, two different which enhanced Littrow-Blaze orders are generated for incident la light generated by two different wavelengths, and a process to determine the geometric parameters for such a grid. Varia tion and modifications of the grids described herein, according to the Invention, will undoubtedly result for the person skilled in the art. Accordingly  the foregoing description is intended to be illustrative and not restrictive be considered.

Claims (16)

1. A flat or planar reflection diffraction grating, which an order of triangular grooves formed by cutting it most and second facets, with the facets at such an angle are inclined to the plane and the arrangement has such a periodicity, that for laser-generated light of two different wavelengths with ver different spreading order numbers, the highest spreading Beu order for each wavelength is "blazed" and the next higher order is not spreading, so that every "blaze" order is strengthened. 2. A grid according to claim 1, wherein the grid is an Echelle grid. 3. A grid according to claim 1, wherein the "Blaze" order numbers are greater than 50. 4. A grid according to claim 1, wherein the grid has an r value between has about 2 and about 6. 5. A grid according to claim 1, wherein the ge by the vertex closed angle is approximately between 80 ° and 90 °. 6. A grid according to claim 1, wherein one of the first and second Fa cetten is shorter than the other of the facets, with the angle between the shorter of the first and second facets and the lattice plane between is approximately 75 ° and approximately 85 °. 7. A grating according to claim 6, wherein the facet angle is between 78 ° and is 81 °. A grid according to claim 1, wherein the frequency of the grooves is between about 0.083 and 0.107 grooves per micron.   A grid according to claim 8, wherein the frequency is between approximately 0.093 and 0.095 grooves per micrometer. 10. A grating according to claim 1, wherein at least one of the two lights radiate from two different wavelengths is emitted by one Krypton Flourin Excimer Laser. 11. A grating according to claim 1, wherein at least one of the two lights radiate the two different wavelengths through an argon fluorine Excimer laser is emitted. 12. A grid according to claim 1, wherein the reinforced "blaze" order for a wavelength of 193.3 nm is 108 and for a wavelength of 248.4 nm is 84. 13. A grid according to claim 1, wherein the grid is for testing upper area flatness with an incident beam angle within one degree of the intended used incident beam angle is suitable, the one falling beam has a wavelength of 632.8 nm. 14. A planar reflective Echelle diffraction grating for enhanced "blaze" order diffraction from a laser-emitted light of two different wavelengths, which comprises:

• a) a groove frequency of approximately 0.09413 grooves per micron;
• b) a facet angle of approximately 80 ° measured with respect to the plane of the grating; and
• c) an angle of approximately 85 ° enclosed by the crest of the groove.

15. A method of forming an Echelle diffraction grating for use in a Littrow orientation around laser emitted light of first and second wavelengths λ ₁ and λ ₁ at first and second amplified "blaze" diffraction angles θ _B λ ₁ and Bend θ _B λ ₂ , and has the following steps:

• a) selecting a facet angle between approximately 75 ° and approximately 85 ° and a vertex angle between approximately 80 ° and 90 ° and assigning λ ₁ to the shorter of the first and second wavelengths;
• b) selecting a range of full-line order numbers m ₁ for the shorter wavelength;
• c) determining values for m ₁ and m ₂ in this range for which the values of the ratio of θ _B λ ₁ / θ _B λ _{2 are} closest to 1.0;
• d) determining values for the groove frequency f for each order m is _1, according to the equation f = 1 / {(m ₁ +1) λ ₁ - m ₁ λ _1/2}, where the next higher order does not propagate;
• e) selecting a value for the groove frequency f for which there is the smallest positive difference between the diffraction angles θ _B λ ₁ and θ _B λ ₂ ; and
• f) Embossing or "rolling" of a grid substrate in order to form a grid master according to the selected values for the facet angle, apex angle and groove frequency.

16. A method according to claim 14, further comprising the step of adding rens a "bias" constant or an impact constant to the be has correct values of frequency f.